This invention relates to a serial-type A/D converter comprising folding circuit cells connected in cascade.
Various types of A/D converter for conversion of analog values into digital values are available of which one is the serial-type A/D converter. Especially, a serial-type A/D converter adapted for the A/D conversion into a Gray code has folding circuit cells connected in cascade as shown in FIG. 2, each cell having an input/output characteristic as graphically shown in FIG. 1. The input/output characteristic, depicted as a solid curve in FIG. 1, is obtained by selecting higher level portions from output waveforms, complementary to each other, of a differential amplifier which exhibits linear output characteristics with respect to input signals. The thus obtained folding circuit cell has a minimum output level at an input threshold voltage V.sub.TH1. The output of this cell a is then inputted to a cell of subsequent stage having a preset input threshold voltage V.sub.TH2, so that the initial input signal can be folded gradually. In FIG. 2, reference numeral 1 designates an analog signal, 2A to 2C folding circuit cells, 3 a threshold voltage, 4A to 4D comparators, and 5A to 5D converter output terminals. The analog signal 1 is folded each time it passes through each cell thereby producing a voltage having a polarity relative to that of the threshold voltage which is determined by the signal at the input point of each cell. Thus, the signal is compared with the threshold voltage at the respective comparators 4A to 4D at the input point of each cell to provide converted outputs at the respective output terminals 5A to 5D.
In the serial-type A/D converter as described above, each of the folding circuit cells is required to have sufficiently accurate gain, offset voltage, linearity and threshold voltage.
A prior art circuit which can be used as the folding circuit cell 2A, 2B, . . . , or 2C is exemplified in FIG. 3. The circuit comprises an input terminal 6, a terminal 7 applied with a threshold voltage from a source 2, transistors 8A and 8B constituting a differential amplifier, constant current sources 9A and 9B, a resistor 10 for linear conversion of the input voltage, load resistors 11A and 11B, transistors 12A and 12B for production of the folded characteristic, a current source 13, an output terminal 14, and a power supply 15. When the voltage between the terminal 6 and 7 is V.sub.i, currents in the current sources 9A and 9B are I.sub.s, the resistor 10 has a resistance of R.sub.d, the current flowing through the resistor 10 is I.sub.d, the load resistors 11A and 11B have a resistance of R.sub.L, and the output voltage is V.sub.L, the differential input voltage V.sub.i is given by ##EQU1## where ##EQU2## k is Boltzmann's constant, T is temperature and q is the magnitude of the electronic charge; and the output voltage is given by EQU V.sub.L =.alpha..multidot.R.sub.L (I.sub.s +I.sub.d) (2)
where .alpha. is the grounded base current amplification factor of the transistors 8A and 8B. However, since R.sub.L .multidot.I.sub.s does not vary to act as a bias component in the output voltage V.sub.L, assuming that the output voltage obtained by subtracting R.sub.L .multidot.I.sub.s from the output voltage V.sub.L is V.sub.L ', EQU V.sub.L '=.alpha..multidot.R.sub.L I.sub.d ( 2)'.
If the condition, ##EQU3## is valid there results a gain K which is expressed as ##EQU4##
Incidentally, in order to attain high resolution of the serial-type A/D converter, as shown in FIG. 2, K=2 must be satisfied. To this end, .alpha..multidot.R.sub.L =2R.sub.d is needed but this matching condition is difficult to achieve because of the difference in resistance between the load resistor 11A or 11B and the linear conversion resistor 10. Further variations in the current amplification factor .alpha. are large making it difficult to make .alpha. a constant.
In addition, in order for the condition, ##EQU5## to be satisfied, I.sub.d &lt;&lt;I.sub.s must be met and at the same time, R.sub.d must be increased, followed by an increased R.sub.L and consequently an increased R.sub.L .multidot.I.sub.s which disadvantageously reduces utilization efficiency of the output voltage.
In addition to the above disadvantages, there occurs an error at the folding portion with the prior art folding circuit cell. More specifically, when the output potential difference between the load resistors 11A and 11B is .DELTA.V, an error voltage .epsilon.(V) in the output voltage appearing at the output terminal 14 is written as ##EQU6## where ##EQU7##
From equation (4), there result EQU .epsilon..perspectiveto.0 for .DELTA.V&gt;&gt;V.sub.T ( 5A) ##EQU8## Equations (5A) and (5B) indicate that, at room temperature, the error voltage at the folding portion becomes about 18 mV which cannot be negligible when constructing the serial-type A/D converter with a resolution of 5 bits or less.
Accordingly, the prior art serial-type A/D converter is partly advantageous in that its construction is simple and can be materialized on a relatively small scale, but the A/D converter is partly disadvantageous in that its resolution is of 5 to 6 bits at the most for the reasons described above, thus preventing highly accurate A/D conversion.